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# Homework Help Watch

1. Hi there, I'm struggling on these two questions. Any help would be very much appreciated

1. A compound of Na, S and O with molar mass 174 is 26.4 % Na and 36.8 % O. An impure 5.00 g sample of the compound decomposed completely to give 593 cm3 of SO2 gas at RTP.
Calculate the formula of the compound and the percentage purity of the sample by mass, showing all the steps in your reasoning.
I've figured out that the empirical formula for the pure sample is Na2s2o4, and that the moles of so2 are 0.024. I got stuck after that

2.Solid X is a hydrated chromium oxide. 12.80 g of X were heated gently to give a constant mass of 8.69 g of anhydrous metal oxide containing 68.4 % chromium. Deduce the formula of X explaining all the steps in the calculation.
I've figured out the empirical formula for the anhydrous metal oxide is Cr203.

Thanks very much for any help x

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3. (Original post by Krakykelly)
Hi there, I'm struggling on these two questions. Any help would be very much appreciated

1. A compound of Na, S and O with molar mass 174 is 26.4 % Na and 36.8 % O. An impure 5.00 g sample of the compound decomposed completely to give 593 cm3 of SO2 gas at RTP.
Calculate the formula of the compound and the percentage purity of the sample by mass, showing all the steps in your reasoning.
I've figured out that the empirical formula for the pure sample is Na2s2o4, and that the moles of so2 are 0.024. I got stuck after that

2.Solid X is a hydrated chromium oxide. 12.80 g of X were heated gently to give a constant mass of 8.69 g of anhydrous metal oxide containing 68.4 % chromium. Deduce the formula of X explaining all the steps in the calculation.
I've figured out the empirical formula for the anhydrous metal oxide is Cr203.

Thanks very much for any help x
The first question has already been dealt with in a previous post (do a search). The question is not possible using the information given.

The second question you seem to have answered. What's the problem?
4. (Original post by charco)
The first question has already been dealt with in a previous post (do a search). The question is not possible using the information given.

The second question you seem to have answered. What's the problem?
As for the second question, I've figured out the formula for the anhydrous metal oxide but not for the hydrated chromium oxide (x).
5. (Original post by Krakykelly)
As for the second question, I've figured out the formula for the anhydrous metal oxide but not for the hydrated chromium oxide (x).
OK

(Original post by Krakykelly)
Solid X is a hydrated chromium oxide. 12.80 g of X were heated gently to give a constant mass of 8.69 g of anhydrous metal oxide containing 68.4 % chromium. Deduce the formula of X explaining all the steps in the calculation
The mass loss is due to water. Divide this by the Mr of water to get moles loss.

Work out the moles of anhydrous Chromium oxide to give you a ratio of moles of chromium oxide to water. This is the formula.
6. (Original post by charco)
OK

The mass loss is due to water. Divide this by the Mr of water to get moles loss.

Work out the moles of anhydrous Chromium oxide to give you a ratio of moles of chromium oxide to water. This is the formula.
That's great
Thank you very much

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